Stopping power is basically correlated with mass.

Keep adding layers until you get to 1.3 feet of lead and it’ll work.

Isn't that more like how you make bombs than armors more effective - with backside spalling and secondary fragmentations?

The point is that shielding turns a single high energy particle that would otherwise strike and probably destroy a single transistor, into a veritable spray of lower energy particles causing bit flips or worse all over the circuit. This spray of particles can be stopped... with 1.3 feet of lead shielding.